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Evaluating the Environmental Impacts of a Nano-Enhanced Field Emission Display Using Life Cycle Assessment: A Screening-Level Study Venkata K. K. Upadhyayula,† David E. Meyer,‡,* Mary Ann Curran,‡ and Michael A. Gonzalez‡ †

Oak Ridge Institute of Science and Education, MC-100-44, PO Box 117, Oak Ridge, Tennessee 37831, United States United States Environmental Protection Agency, Systems Analysis Branch, National Risk Management Research Laboratory, 26 West Martin Luther King Drive, Cincinnati, Ohio 45268, United States

‡

S Supporting Information *

ABSTRACT: Carbon nanotube (CNT) field emission displays (FEDs) are currently in the product development stage and are expected to be commercialized in the near future because they offer image quality and viewing angles comparable to a cathode ray tube (CRT) while using a thinner structure, similar to a liquid crystal display (LCD), and enable more efficient power consumption during use. To address concerns regarding the environmental performance of CNT-FEDs, a screening-level, cradle-to-grave life cycle assessment (LCA) was conducted based on a functional unit of 10 000 viewing hours, the viewing lifespan of a CNT-FED. Contribution analysis suggests the impacts for material acquisition and manufacturing are greater than the combined impacts for use and end-of-life. A scenario analysis of the CNT paste composition identifies the metal components used in the paste are key contributors to the impacts of the upstream stages due to the impacts associated with metal preparation. Further improvement of the manufacturing impacts is possible by considering the use of plant-based oils, such as rapeseed oil, as alternatives to organic solvents for dispersion of CNTs. Given the differences in viewing lifespan, the impacts of the CNT-FED were compared with a LCD and a CRT display to provide more insight on how to improve the CNT-FED to make it a viable product alternative. When compared with CRT technology, CNT-FEDs show better environmental performance, whereas a comparison with LCD technology indicates the environmental impacts are roughly the same. Based on the results, the enhanced viewing capabilities of CNT-FEDs will be a more viable display option if manufacturers can increase the product’s expected viewing lifespan.

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INTRODUCTION Field emission displays (FEDs) are an emerging class of electronic displays with potential to generate excellent image quality using a structure that is flatter than any other flat panel display.1 Field emission is defined as the quantum tunneling of electrons generated by a conducting solid surface in vacuum when placed under the influence of an intense external electric field2−4 The structure of an FED consists of a cathode and anode substrate. The cathode acts as an electron generating source and contains millions of electron-emitting tips made of conducting material whereas the anode is coated with colored phosphors.5 When the external electric field is applied, the electrons emitted from the nanodimensional cathodic tips traverse the evacuated space, striking and exciting the phosphor coated anode surface to produce an image. The working principle of FEDs is similar to that of cathode ray tubes (CRTs), with the main difference being the electron emission in FEDs is based on several individual nanoscopic electron guns as opposed to the single large electron gun used in CRTs.2 The field emission efficiency of an FED depends on the electron This article not subject to U.S. Copyright. Published 2013 by the American Chemical Society

generating ability of the cathode and ultimately the shape of the nanodimensional tips fabricated on the cathode substrate. A needle-shaped tip having a sharp protruding edge is preferred over a rounded, blunt-surfaced tip because the former has the ability to generate and localize a dense electron cloud around its surface and edges.2 By incorporating millions of cathode tips on the substrate surface, FEDs are able to utilize a large area for field electron emission to generate high quality images while maintaining a wide angular vision field comparable to CRTs, a consumer-preferred flat structure and a weight-to-size ratio less than CRTs and liquid crystal display (LCD) devices (weight to size ratios are in order of CRT (0.89) > LCD (0.11) > FED (0.02)).2 Additionally, manufacturers claim that FEDs are the most energy efficient displays (10−12 W),6,7 even when Received: Revised: Accepted: Published: 1194

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Figure 1. Flow diagram showing major stages in the product life cycle of CNT-FED.

amounts of organic solvents (e.g., isopropanol) and metal powders (e.g., silver powder).14 Use of these chemicals may create solvent rich and/or metal-rich waste streams with large human health and ecotoxicity impact potentials.16,17 Furthermore, recent toxicological studies have shown CNTs themselves can lead to adverse human and ecological effects, increasing the need to track CNTs throughout their life cycle.18−26 The application of life cycle assessment (LCA) to emerging CNT products can provide the needed insight to holistically address concerns related to CNT toxicity as well as other potential environmental and health impacts.26 In a previous study examining the life cycle impacts of a CNT-FED in Europe, Bauer et al.27 reported the highest impacts occurred in the use stage. They also attributed the greatest impacts in the manufacturing phase to the fabrication of the CNT cathode substrate. This identification provides an example of an opportunity for research to be conducted on the reduction of impacts associated with the manufacturing phase. Another key finding of the study suggests CNT-FEDs are expected to have a significantly improved environmental performance when compared to CRTs and LCD displays. Although this is encouraging from a market commercialization standpoint, it should be noted the potential impacts could be underestimated because certain stages in the life cycle, particularly the end-of-life, were not modeled completely due to critical data gaps such as the release of CNTs.27 In this paper, the results of a screening-level, cradle-to-grave LCA of a hypothetical CNT-FED are presented. The screeninglevel LCA establishes baseline data which can be compared with existing display types. It can also help identify key hotspots within the product life cycle that are responsible for major impact contributions. This work is intended to provide an updated baseline to the Bauer study by considering the latest CNT-FED prototype designs. Additionally, this study applies a U.S.-based life cycle inventory, which builds on the concepts developed in the European study. The environmental performance of a CNT-FED is quantified and compared with results for both a LCD and CRT display having equivalent display characteristics. An uncertainty analysis is performed on the CNT-FED results to understand the level of uncertainty introduced by the largely varying chemical composition

compared to advanced LCD panel monitors (22−25W) containing LED backlights.8 The fabrication of nanoscale cathode tips has proven to be extremely challenging because the electric field required for field emission generates immense heat capable of altering the tip structure. For example, initial FED prototypes used molybdenum (Mo) microscale tips as the cathode source, but were unable to be commercialized because the tips quickly become deformed and eventually lose emission efficiency.9 Interest in FEDs was renewed with the discovery of carbon nanotubes (CNTs) in 1991, 10 and subsequent studies recognizing their ability to provide high emission current densities at low applied electric fields.9 CNT-enabled FED technology offers three principle benefits. The high chemical stabilities of CNTs allow them to survive in extremely intense electric fields without suffering the quick deformation that plagued the Mo-tipped emitters. Because of their theoretically defect-free surface structure (i.e., one-dimensional and seamless cylindrical structure), CNTs are not susceptible to dangling bonds and self-diffusion of surface atoms, common causes for instability in metallic emitters. A highly concentrated electric field can be localized at the tip surface of CNTs and facilitate FEDs in achieving high field emission intensities.9 Although the research and development of CNT-FED technology began a decade ago, commercialization of these devices has been delayed based on alignment issues with CNTs in early prototype versions using a chemical vapor deposition (CVD) based vertical alignment growth process.11−13 To rectify this, the latest prototype designs use screen printing to apply a CNT conductive paste to the cathode substrate and achieve a uniform alignment.11,13,14 Small commercial models based on these prototypes have been already deployed in hospitals and other public places in some parts of the world.15 This is a big step toward commercialization of full scale CNTFEDs in the near future. While commercialization of CNT-FEDs can be accomplished technologically, concerns regarding the potential environmental and human health impacts that may result from their manufacture, use, and disposal should be considered prior to large-scale deployment. For example, preparation of the CNT cathode paste for screen printing will require significant 1195

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•dispersion is accomplished by adding CNTs and isopropanol (IPA) solvent according to U.S. Patent (US 7, 040, 948, B2) information45 (1 g CNTs dispersed in 300 mL of IPA and mixture is milled) •CNT paste formulation is obtained from literature11,13,14,46,47 •the amount of CNT conducting paste needed to screen print one cathode substrate is 100 g13 •the amount of glass and indium tin oxide (ITO) powder is the same as the ITO consumed in LCD production30 •sputtering of ITO on a glass substrate is based on Ecoinvent data32 •UV irradiation for 5−10 min11 •surface treatment with a chemical solution (3−10 wt % alumina and 5−20% organic polymer binder in DI water)11

dispersion of acid purified CNTs in organic solvent

CNT conducting paste formation

screen printing CNT paste on ITO coated glass substrate

post treatment of screen printed CNT cathode substrate

Final Assembly of CNT-FED assembling cables, printed board to vacuum sealed FED

Vacuum Sealed Module of CNT-FED vacuum sealing of CNT cathode and phosphor anode substrates

•LCI data is based on the work of USEPA30 •housing and electronic component data is the same as LCDs30

•LCI data is based on patent literature for high vacuum sealing of flat panel displays49

•phosphors composition contains ZnS, Cu, and Ag according to U.S. Patent48 •the phosphor coating applied to the indium tin oxide (ITO)-coated glass substrate is the same as LCDs30

•the purification solution is a 3:1 (v/v) mixture of concentrated H2SO4 (98%) and HNO3 (68%)44

acid purification of CNTs

Fabrication of Phosphor Anode Substrate production of phosphors and sputtering them onto the substrate

•the C2H2/H2 feedstock is assumed to be preheated as described in Plata et al.41 •The catalyst is used for a single batch and discarded as solid waste and in a hazardous waste landfill •the quantity of CNTs produced per batch is 1 kg •the carbon deposition yield and synthesis reaction yield are assumed to be 30% based on a realistic estimate for bulk production of literature CNTs synthesis via a catalytic CVD process42,43 •losses of recirculation gas streams and purge gases are negligible •atmospheric emissions from a CVD reactor using a C2H2 feedstock are obtained from Plata et al.41

•catalyst composition: (20:20:60) wt% Ni:Fe:Al2O3. For Ni and Fe metals, NiSO4 and FeSO4 metal salts are used as precursors33,35 •NiSO4 is assumed to be prepared via reacting Ni metal with dilute H2SO440 •catalyst preparation process yield: 90%

major assumptions

synthesis of CNTs

stages Fabrication of CNT Cathode Substrate production of metal catalyst needed for CNT Synthesis

Table 1. Assumptions Made When Modeling CNT-FED Manufacturing

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distributions achieved using bimetallic catalysts,33 which result in better structural quality and crystallinity.37,38 Acetylene (used in conjunction with H2 gas) is considered to be the most preferred carbon precursor choice for field emission applications requiring flawless CNT structures. 39 The quantity of metal catalyst required for synthesis of the CNTs was calculated using a specified CDY according to eq 1 taken from Leone.34

reported for making the CNT paste as part of the fabrication of the cathode substrate. Uncertainty analysis is also performed on CRT and LCD results by incorporating minimum and maximum use phase energy values for the respective displays reported by Lawrence Berkeley National Laboratories.28 Furthermore, a combination of contribution and scenario analysis is applied to key aspects of the manufacturing process to better understand to what extent process improvements can reduce the total environmental impacts.29

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⎡M ⎤ CDY = ⎢ cd ⎥ × 100 ⎣ Mcat ⎦

LIFE CYCLE ASSESSMENT METHOD Goal and Scope Definition. The goal of this study is to perform a screening-level LCA to measure the environmental performance of a hypothetical CNT-FED desktop display that is manufactured, used, and disposed of within the continental U.S. and compare the environmental impact assessment results with those reported in the literature for both a CRT and an LCD display with equivalent functional characteristics.30 The functional unit for this LCA is 10 000 h of viewing, which is currently the estimated viewing lifespan of the display.6 For the 10 000 viewing hours, it is further assumed the display is cycled from full power mode to standby mode for 50% of the time (5000 h each). This assumption is based on a consideration that the average duty cycle typically used by component manufacturers to test the components of an electronic display is 50% time in full power mode and 50% standby mode.30 Constructing the Life Cycle Inventory for a CNT-FED. The typical product life cycle stages for a CNT-FED are shown in Figure 1. In this study, data sets for the first two life cycle stages (material acquisition and manufacturing) are represented by the fabrication of a CNT-cathode substrate, fabrication of a phosphor anode substrate, vacuum sealing of the cathodeanode assembly and final product assembly (cable wiring, printed board, display housing, etc.). A detailed explanation of how these processes were modeled is available in the Supporting Information. Data sets for use and end-of-life complete the product life cycle. A discussion of key assumptions and calculations used during construction of the life cycle inventory follows. Data Sources. Using the SimaPro LCA software package (version 7.3), the life cycle inventory (LCI) data for a CNTFED manufacturing stage was assembled using patent and academic literature. Unless otherwise noted, all data for ancillary processes (raw materials processing, utilities, end-oflife treatment, etc.) are U.S.-modified secondary data obtained from the National Renewable Energy Laboratory (NREL) US LCI database31 and the Ecoinvent database.32 Particularly the energy consumption of these data sets is calculated using the U.S. national energy grid mix. Acquisition and Manufacturing. The assumptions used to model the LCI for the manufacture of a CNT-FED are presented in Table 1. For the purpose of this work, the stages of material acquisition and product manufacturing will be collectively referred to as “manufacturing”. The LCI data corresponding to production of the metal catalyst and synthesis of CNTs are based on the carbon deposition yields (CDYs) and synthesis reaction yields (SRYs) reported in Fazle-Kibria et al.,33 Leone,34 and Liu and Harris35 for the use of acetylene with a bimetallic nickel and iron catalyst supported on aluminum oxide. CNTs produced via bimetallic catalyst systems have shown the ability to generate higher field emission efficiencies than those synthesized via monometallic catalytic systems.36 This can be attributed to the smaller CNT diameter

(1)

where Mcd is the weight of carbon product obtained after synthesis and Mcat is the weight of dry catalyst fed to the reactor. The SRY value for a corresponding CDY value can be used to calculate the quantity of acetylene (C2H2) required for synthesis according to eq 2 taken from Logeswari et al.50 ⎡ M ⎤ SRYofC2H 2 = ⎢ cd ⎥ ⎢⎣ MC2H2 ⎥⎦

(2)

The data for acid purification of CNTs is based on the results of Wang et al.44 who reported a 99.9% yield for CNT purification. The unpurified CNTs are annealed at high temperature (2000 °C) for 30 min to prevent wall layer damage and then soaked in a concentrated acid mixture (H2SO4/HNO3). This method produces untangled, undamaged, high purity CNTs that are easily dispersed in organic solvents.44 Solvent dispersion is modeled according to the procedure described in patent51 and academic52 literature. Preparation of the CNT paste is a critical step in the fabrication of the CNT cathode substrate. This stage can introduce a large degree of variability in the environmental performance because the composition of the paste will vary based on the type of chemicals (i.e., organic binders, photosensitive vehicles, metals, etc.) used. Although extensive literature is available for the preparation of a CNT conductive paste, the research of Bak et al.,46 Jin et al.,11 and Kim et al.14 was used to model this process because this compilation of work is current (2−4 years old), includes discussions of the selection and compositions of chemicals, and provides a detailed description of the screen printing and curing methods to enable completion of the LCI for this process. In order to better simulate a real-world product, the baseline LCI for a CNT-FED was constructed by assuming the processes studied in the three sources are used to produce the CNT paste composition established in U.S. Patent 20080004380A1.11 Additional CNT paste compositions11,14,46 were considered using scenario analysis to examine the influence of this material on the overall impacts. The LCI describing the preparation of the phosphor anode is based on a well-established process.48 The vacuum process used to seal the CNT cathode and phosphor anode substrates is based on US Patent 6,554,672 B2,49 which describes a procedure for vacuum sealing flat panel displays. Finally, the housing assembly LCI is assumed to be the same as that of LCD reported by U.S. EPA report on desktop displays.30 Use and End-of-Life. For this study, the assembled CNTFED unit was assumed to be used by consumers for 10 000 h (viewing lifespan). Since no performance-based information for the useful life of a CNT-FED is currently available, the method used to compute the useful life of a LCD display has been adapted from a USEPA report on desktop computer displays.30 1197

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Assumptions Made Regarding Use Of Electronic Displays

Table 2. Assumptions Made When Modeling Display Use and End-of-Life

Other assumptions relevant to modeling display use and endof-life are given in Table 2 and include assumptions regarding LCDs and CRT displays for eventual impact comparison. For this screening level assessment, all CNT-FEDs are assumed to be landfilled. Although the risk of release of CNTs during disassembly for recycle/reuse is expected to be high,53 accepted CNT impact characterization factors for human health and ecotoxicity have yet to be established. Incineration was not considered because the final disposal is assumed to be done within the United States where incineration is not a commonly used method for end-of-life management. Furthermore, the complete environmental effects of incinerating products with embedded nanomaterials have to be thoroughly quantified before considering incineration as an end of life management option. For example, a study examining CNT-enabled batteries suggests CNTs will retain their structure in the temperature zones at which the current incinerators operate and implies the energy consumption, and therefore the carbon dioxide emissions, might need to be increased to accommodate them.54,55 Incineration of nanomaterial-embedded products at low temperatures may also affect recycling operations because certain recycled materials may get contaminated with nanomaterials escaping the incineration process.54 Although recycling may prove to be a beneficial option for nanoproducts, no data currently exists to support modeling this scenario with regard to processes and material flows.54 Thus, landfilling excluding the environmental release of CNTs presents the least uncertain scenario. The LCD and CRT displays are also assumed to be disposed using 100% landfilling to provide a valid comparison with the CNT-FED life cycle. LCI Summary. The key inputs for the CNT-FED model are given in Table 3. Supporting calculations used to model manufacturing processes and inventory for upstream processes (electricity production, solvent production, etc.) are provided in the Supporting Information. Comparison with CRT and LCD Desktop Displays. Cradle-to-grave LCIs for both LCD and CRT displays are based on the U.S. Environmental Protection Agency report EPA 744-R-01-004a30 and available data in the USLCI database (as accessed through SimaPro). For the comparison of display types the functional unit is reset as the viewing lifespan of the LCD (45 000 h) because it provides the longest life and is the most commonly used display type. Therefore, an equivalent product comparison is one LCD product to 3.6 CRTs or 4.5 CNT-FEDs. Impact Assessment. The life cycle impacts of a CNT-FED are calculated by applying the U.S. Environmental Protection Agency’s Tool for the Reduction and Assessment of Chemical and other environmental Impacts (TRACI).56 TRACI is a midpoint impact assessment tool that prioritizes the use of U.S.-based LCI data and impact models.56 The calculated midpoint impacts given by TRACI are global warming potential (GWP, kg CO2 eq), acidification potential (AP, kg SO2 eq), human health impacts from carcinogens and noncarcinogens (HH-C, HH-NC, CTUh), respiratory effects (RE, kg PM2.5 eq), eutrophication potential (EP, kg N eq), ozone depletion potential (ODP, kg CFC-11 equiv), ecotoxicity (ETX, CTUe), smog (kg O3 eq), and fossil fuel depletion (FFD, MJ surplus). The version of TRACI used in this study is TRACI 2.1.,57 and incorporates the USEtox environmental model for characterization of human and ecotoxicity impacts.58,59 Uncertainty Analysis on CNT-FED Results. As noted in Table 3, the composition of the CNT paste given in U.S. Patent

Assumptions Commonly Applied to All Three Displays •the useful life of all the three electronic displays is estimated based on their manufactured life. •viewing lifespan: the amount of time a product lasts before it loses functionality based on product testing of average duty cycles30 •the average duty cycle consists of 50% full power mode and 50% standby mode30 •use phase power is obtained from a US electric grid Assumptions Specif ic To CNT-FED Use •two companies in Taiwan, Giantek Corporation and Teconano, have reported a manufacturing life of 10 000 h for an 18-in. CNT-field emission indicator6,15 •the viewing lifespan includes 5,000 h each of full power mode and standby mode30 •power consumption during full power is 12 W 6, 15 and standby power is assumed to be same as the LCD (1.38 W) given on Lawrence Berkeley National Laboratories (LBL) standby power summary table28 Assumptions Specif ic To LCD Use •the viewing lifespan is 45 000 h with a full power mode of 22 500 h and a standby mode of 22 500 h •power consumption during full power on and standby modes are 27.61 and 1.38 W, respectively.28 For uncertainty analysis the power consumption values taken for LCD are 1.9 W (min) and 55.48 W (max) for full power on mode; 0.37 W (min) and 7.8 W (max) for sleep mode28 Assumptions Specif ic To CRT Use •the viewing lifespan is 12,500 h with a full power mode of 6250 h and a standby mode of 6250 h •power consumption during full power and standby mode are is 65.14 and 12.14 W respectively. For uncertainty analysis, the power consumption values for the CRT are 34.54 (min) and 124.78 (max) for full power on mode; 1.6 W (min) and 74.5 W (max) for standby mode Assumptions Made Regarding End-of-Life For Electronic Displays •CNT-FEDs are 100% landfilled because no data exists to predict and model the effects of CNTs on recycling, reuse, and incineration options •although recycle, reuse, and incineration options exist for LCDs and CRT displays, 100% landfilling is considered to provide a valid comparison to the CNT-FED life cycle •no release of CNTs occurs

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Table 3. CNT-FED Life Cycle Inventory material input

mass (kg)

notes/references Synthesis Of 100 g Of CNT Conducting Paste

acid purified CNTs isopropanol bisphenol (A) (Binder resin) ethyl acrylate (photosensitive vehicle) Glass frit silver (metal powder)

0.03 7.074 0.04 0.02 0.005 0.005

Fabrication Of 1 CNT Cathode Substrate CNT conducting paste from stage 1 0.1 Cr-coated flat glass 0.590 ITO powder 5 × 10−04 epoxy resin 0.40 alumina 0.20 deionized water 1.40 Fabrication of 1 Phosphor Anode Substrate zinc sulfide 0.0188 copper 2 × 10−04 silver 1 × 10−03 flat glass, uncoated 0.59 ITO Powder 5 × 10−04 Vacuum sealing of CNT-FED Module CNT cathode substrate 0.709 phosphor anode substrate 0.629 glass frit 0.098 bentonite 2 × 10−03 deionized water 0.150 Final Assembly of One CNT-FED Unit vacuum sealed CNT-FED module 1.438 chromium Steel Frame 0.1772 copper 0.00484 printed wiring board 0.0684 Integrated circuit IC logic type 0.0128 nylon 0.00984 synthetic rubber 0.00632 polycarbonate 0.500 total CNT-FED manufacturing energy input use stage energy consumption CNT-FED To landfill

U.S. Patent Application 7,648,424 B2.11 Fifteen wt% CNTs, 40% binder resin, 30% photosensitive vehicle, 7.5% frit, and 7.5% metal powder

U.S. Patent Application 7,648,424 B2.11 twenty wt% epoxy resin and 10 wt % alumina in deionized water

U.S. Patent 3,657,142.46 Cu: 0.001−0.5 wt %; Ag: 0.01−0.09 wt %

U.S. Patent Application 6,554,672 B2.

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based on methods in EPA 744-R-01−004a30

73 kWH 66.9 kWH 2.217

based on methods in EPA 744-R-01−004a30 ecoinvent32

expected given two key differences in the life cycle models. First, Bauer’s group assumed a 15-in. CNT-FED consumes half as much power as an LCD,30 whereas this study uses a reported value closer to one tenth of the LCD. Furthermore, this study is based on the latest CNT-FED prototype and involves screen printing for cathode fabrication. The CNT-FED manufacturing stage was further analyzed using contribution analysis to identify hotspots for potential stage improvement. The contributions to the ten impact categories by each of the major manufacturing steps are shown in Figure 3. Among the four manufacturing steps, the phosphor anode fabrication step contributes to the least impact in all nine impact categories whereas the CNT cathode fabrication step is responsible for 59% of the fossil fuel depletion, 47% of the eutrophication potential, 41−43% of the impacts for acid-

7,648,424 B2 was reported using a range of weight percentages.11 The median value of a specified range was used for the baseline model. Therefore, an uncertainty analysis based on the corresponding ranges was conducted in SimaPro using a Monte Carlo simulation with 1000 steps and a 95% level of confidence.

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RESULTS AND DISCUSSION The internally normalized impact scores for the CNT-FED are shown in Figure 2 using contribution analysis to understand the relative contributions of the life cycle stages. The manufacturing stage clearly contributes the most in each impact category. This finding differs from that of Bauer et al.27 who suggest the use phase is the most dominate phase. This difference should be 1199

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Figure 2. Screening-level, cradle-to-grave life cycle impact assessment results for a CNT-FED (viewing lifespan = 10 000 h).

Figure 3. Contribution analysis of CNT-FED manufacturing using internally normalized impact scores.

mean impact score are included for the CNT-FED to account for the uncertainty of paste composition and ecoinvent process data whereas the error bars (1 standard deviation) for the LCD and CRT describe the variability in reported power use and uncertainty in ecoinvent process data. Of the three display types, the CRT has the least desirable environmental performance in all 10 impact categories studied because of its short viewing lifespan combined with its high use-phase energy consumption. Although the viewing lifespan of a CNT-FED is shorter than that of a CRT display, its overall performance is comparatively better because of less impactful manufacturing processes and reduced energy consumption during use. The overall environmental impact of the CNT-FED is also less than the LCD in 9 out of 10 impact categories, the exception being its ozone depletion potential. The ecotoxicity impact of the CNT-FED is 62% lower than LCD, with 36% of its potential impact directly related to the use of silver powder and IPA

ification, respiratory inorganics and smog, 38% of the GWP, 36% of the ecotoxicity impact, 34% of the human health impact from carcinogens, 24% of the human health impact from non carcinogens, and 31% of the ozone depletion potential. Within the CNT cathode fabrication step, the CNT paste formulation step (details are provided in the Supporting Information) is responsible for 87−93% of the impact in all categories. Assembly of the housing and electronic circuits accounts for approximately 25% or greater of all manufacturing impacts (with the exception of eutrophication potential) because of the use of printed wiring board (PWB) material involving processes such as soldering.30,60 The comparison of the life cycle impacts for the CNT-FED display with the CRT and LCD displays is shown in Figures 4. The results are again internally normalized to the highest impact score among the three displays for that particular impact category. Errors bars representing 1 standard deviation of each 1200

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Figure 4. Comparison of impact results with uncertainty analysis for three electronic displays based on a functional unit of 45 000 h of viewing lifespan (1 LCD = 3.6 CRTs = 4.5 CNT-FED).

resulting use-phase power ratings are comparable to emerging CNT-FEDs. However, no data could be located to support comparison with these technological enhancements for the current study. Scenario Analysis of CNT Paste Formulation. Based on contribution analysis (Figures 2 and 3), manufacturing is the dominant life cycle stage and is most significantly affected by the assembly of the housing and electronic components in 5 of the 9 impact categories, whereas the CNT cathode fabrication step has greater impacts in the remaining four categories (GWP, acidification, respiratory inorganics and smog formation). Because the LCI data describing housing and electronic components is common to both CNT-FEDs and LCDs, the CNT cathode fabrication step presents the best option for improving the environmental performance of CNTFED technology specifically. Within the CNT cathode fabrication step, formulation of the desired paste composition causes 87−93% of the impacts in all 10 categories (see Supporting Information). Thus, a scenario analysis was conducted on the manufacturing stage to identify how optimization of the CNT paste can be used to improve the environmental performance of a CNT-FED. As opposed to uncertainty analysis examining changes in impacts based on ranges of values for the existing paste components, scenario analysis focuses on suitable alternative pastes which maintain the desired functionality. Four alternate CNT paste compositions were considered based on the methods reported by Bak et al.46 and Jin et al.11 The key differences in the selected paste compositions are summarized in Table 4 with detailed descriptions of preparation methods provided in the Supporting Information. The effects of changing the CNT paste composition, or more specifically, the effect of changing two principal ingredients in the CNT paste, the solvent (used for CNT dispersion) and the conductive metal/metal oxide powder (used to impart the conductive properties to CNT paste), on the overall environmental performance of the CNT-FED is shown in Figure 5. Of the five paste formulation methods considered, Scenario-0 generated the maximum impact across all nine categories.

solvent during manufacturing of the CNT cathode substrate. Among other impacts, the global warming potential, acidification potential, smog and respiratory effects of the CNTFED are 23−27% lower than the LCD values. Fossil fuel depletion, human health impacts and the eutrophication potential are only 8−11% lower than the LCD, which may not be statistically relevant based on the reported standard deviations. When considering eutrophication, direct emissions from nitrogen and phosphorus use during the panel manufacturing and housing assembly processes are the main contributors for the LCD.30 The eutrophication potential for the CNT-FED is a reflection of both the use of isopropanol solvent as a dispersant during cathode manufacturing and the housing assembly process, which contributes to 47% of the total EP impact. The mean value for ozone depletion potential of the CNT-FED is 28% higher than the LCD because of the use of silver metal during paste synthesis. The CNT-FED and LCD both yield impacts 52−82% lower than the CRT display for nine of the ten categories, with the 19−28% reduction in eutrophication potential as the lone exception. It is important to note that the impacts of the LCD and CRT display were modeled using inventory data that is more than a decade old. Although CRT display technology has remained stagnant, considerable improvements have occurred in LCD technology since the USLCI module was created. For example, the power consumption of the device has been significantly reduced, resulting in impact scores only marginally higher for 8 out of 10 impact categories. Considering the significant reductions in the use stage power consumption of LCDs in recent years, the differences in life cycle impacts generated by a CNT-FED and LCD may not be statistically meaningful based on the uncertainty analysis. Thus, the performance of CNT-FEDs may only prove beneficial when compared to LCDs if the viewing lifespan can be increased. This will be a more interesting and meaningful comparison when emerging LCD technologies are included. Specifically, the use of light emitting diode (LED) and organic LED (OLED) technology combined with “ultrathin” case specifications may support better environmental performance, especially if the 1201

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Bak et al.46

Changing the metal from silver to zinc (Scenario-1) reduced the associated impacts by a minimum of 0.4% for eutrophication and a maximum of 5.2% for smog, with a range of 1−4% reduction for the remaining impacts. These findings suggest metal selection based on pure metals (Ag or Zn) does not offer enough opportunity for impact minimization. The influence of solvent selection can be observed by comparing Scenario-2 with Scenario-0. IPA is classified as a volatile organic compound (VOC) and is known to cause ozone depletion, particulate matter and smog formation and increased global warming potential during its manufacturing. The use of a dispersant derived from a renewable feedstock (rapeseed) in Scenario-2 as opposed to a fossil-based solvent (IPA) offers a maximum 36% reduction in fossil fuel depletion, followed by a 26% reduction in eutrophication potential, and a 10−14% reduction for the remaining impact categories. The exception is human health impact from non carcinogens because the main driver, the extraction, purification, and use of silver powder, is present in both scenarios. The results suggest continued development of CNT-FEDs will benefit greatly from the application of green design principles for solvent use to achieve an optimal environmental profile. The impact scores for Scenario-3 further examine the effects of metal selection by substituting a metal oxide (TiO2) for Ag powder. It offers a maximum of 21% reduction in fossil fuel depletion, with a 17% reduction in eutrophication potential, 12−13% reduction in acidification potential, respiratory effects and smog, and approximately a 10% reduction in the remaining impact categories. There is a sizable reduction in the ecotoxicity impact when TiO2 is used in place of Ag or Zn powder because the potential toxic effects of Zn and Ag compounds are higher than TiO 2 for selected aquatic species.61−63 Similarly eutrophication is lowered when TiO2 is used in the paste composition. This change is agreeable with the fact that the photocatalytic activity of TiO2 can cause inhibition of algal growth,64 whereas Zn and Ag can actually promote its growth under certain environmental conditions.65,66 Finally, there is a noticeable reduction in ozone depletion in scenario 3 because the photodissociation potential of TiO2 is less than ZnO,67 a compound formed when zinc ore (ZnS) is mined to obtain Zn metal.68 Of the four scenarios considered, Scenario-4 is the most effective alternative for reducing the life cycle impacts because it offers the largest reduction (relative to Scenario-0) in all of the impact categories. The reduced environmental impacts should be expected given Scenario-4 uses rapeseed oil for the dispersant and titanium dioxide (TiO2) powder instead of silver to enhance the conductivity of the CNT paste. In Scenario-4, the maximum reduction is observed for fossil fuel depletion (39%), followed by eutrophication potential (30%), ozone depletion potential and human health impacts from non carcinogens (20% each), human health impacts from carcinogens (11%), and finally the remaining impact categories (16−18%). Literature suggests that other metal oxide powders, including silicon dioxide, tin oxide, and zirconium oxide can be used to make suitable CNT pastes.46 Additional assessments of pastes using these materials will provide a better understanding of which metal sources provide the least impact. This information can be combined with solvent studies and economic data to develop cost-effective paste alternatives with enhanced environmental performance. .

acid purified CNTs: 15 g; epoxy resin 44 g; methyl acrylate 35g; Silver powder 5 g; anthraquinone 1 g, and rapeseed oil 150 g

acid purified CNTs: 15 g; epoxy resin 39 g; Methyl acrylate 25g; glass frit 5 g;TiO2 15 g; anthraquinone 1g, and IPA solvent 3.537 kg

acid purified CNTs: 5 g; epoxy resin 49 g; methyl acrylate 30g; TiO2 15g; anthraquinone 1 g, and rapeseed oil 50 g

3

4

1

2

•IPA solvent used to disperse CNTs •Zn powder substituted for Ag powder to impart conductive properties •rapeseed oil is substituted for IPA during CNT dispersion •Ag powder used for conductivity •IPA solvent used to disperse CNTs •TiO2 is substituted for Ag powder •rapeseed oil is substituted for IPA during CNT dispersion •TiO2 is substituted for Ag powder

Jin et al.11 •baseline

acid purified CNTs: 30 g; epoxy resin (binder resin) 39 g; methyl acrylate (organic photosensitive vehicle) 20g; glass frit 5 g; Silver powder 5 g; anthraquinone (photo initiator) 1 g, and isopropanol (IPA) solvent 7.074 kg acid purified CNTs: 30 g; epoxy resin 39 g; methyl acrylate 20 g; glass frit 5 g; zinc powder 5 g; anthraquinone 1g, and IPA solvent 7.074 kg 0 (baseline)

reference differences with respect to baseline scenario characteristic features of CNT paste composition scenario

Table 4. Summary of CNT Paste Compositions Examined During Scenario Analysis

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Figure 5. Screening-level, cradle-to-grave impact results for a CNT-FED based on four CNT conducting paste formulations (Scenario 0 (Baseline (IPA & Ag Powder)); Scenario 1 (IPA & Zn Powder); Scenario 2 (Rapeseed Oil & Ag Powder); Scenario 3 (IPA & TiO2); Scenario 4 (Rapeseed Oil and TiO2).

FEDs as a competitive display option with minimal environmental impact. Future work examining display technologies should focus on the development of accurate life cycle inventories for these technologies to allow for more meaningful comparison. Any comparison with CNT-FEDs should be periodically revisited as technologies evolve to provide lower energy requirements and increased viewing lifespans. Based on historic trends for product development, it is reasonable to expect the CNT-FED-to-LCD ratio of 4.5:1 will eventually approach a more favorable 1:1 comparison. Additional limitations of this work arise from the end-of-life modeling assumptions used for this screening-level LCA. To better address this aspect of the life cycle, critical data is first needed to quantify the impact of CNTs on both product durability for landfilling and disassembly and material recovery for potential recycling processes. This data can then be combined with necessary characterization data, including fate, transport and potential toxicity of CNTs, to develop a more accurate end-oflife model. Although significant uncertainty exists, it is encouraging that the end-of-life impacts for the current studies are significantly smaller compared to its manufacturing and use stages. These results may potentially prove to be accurate if the release of CNTs from the display is proven to be negligible under landfill conditions. Ultimately, more detailed LCA studies are needed to better understand how to best attain superior picture quality and environmental performance for desktop devices using emerging technologies.

The application of screening-level life cycle assessment to an emerging product can provide insights about the potential environmental issues which should be considered for large-scale product deployment. The fabrication technology for the CNT cathode considered here is not only applicable to the design of CNT-FEDs, but also has other potential uses such as an electron source generator for X-rays and microscopes, solid state lighting, backlighting for LCDs and in electrostatic scrubbers.69 Thus, the trends identified by contribution and scenario analysis for the CNT cathode fabrication process discussed here can serve as a baseline when evaluating the environmental performance of other similar products and applications mentioned above. The scenario analysis showed a key contributor to the impacts of the material acquisition and manufacturing is the metal components used in the paste formulation because of the upstream impacts associated with metal preparation. For this reason, the use of metal oxides and nonsilver powders is preferred. Similarly, the use of an organic solvent such as IPA contributes significantly to the manufacturing impacts. Therefore, further improvement to the performance of CNT-FEDs is possible by considering the use of renewable oils such as rapeseed oil for dispersion of CNTs. Perhaps the most notable finding in this study is the nearly identical environmental performance of the CNT-FED when functionally compared to low-power LCD technology. With an expected 10 000 h viewing lifespan, 4.5 CNT-FEDs would be required to provide a functional viewing time equivalent to one LCD (45 000 h.). Therefore, any reduction in environmental impacts associated with a single CNT-FED will be negated in applications requiring longer viewing lifetimes. The more desirable picture quality of CNT-FEDs will only gain a competitive edge over LCDs if manufacturers can increase the product’s expected lifespan. As previously discussed, this study does not include recent advances to LCDs such as LED technology, which may improve the impact results for these displays. The inclusion of new technologies is needed to better guide the design of CNT-

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S Supporting Information *

Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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Corresponding Author

*Phone: +1-513-569-7194; e-mail: [email protected]. Notes

The authors declare no competing financial interest. 1203

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ACKNOWLEDGMENTS The U.S. Environmental Protection Agency through its Office of Research and Development funded the research described here. It has not been subjected to full Agency review and therefore does not necessarily reflect the views of the Agency, and no official endorsement should be inferred. This research was supported in part by an appointment of Dr. Venkata K.K. Upadhyayula in the Postdoctoral Research Program at the National Risk Management Research Laboratory, administered by the Oak Ridge Institute for Science and Education through Interagency Agreement No. DW 89- 92298301-0 between the U.S. Department of Energy and the U.S. Environmental Protection Agency.

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